1. Field of the Invention
The present invention relates to a protecting unit for preventing an accident particularly during an extraordinary state from occurring in an electronic instrument using for instance laser.
2. Description of the Related Art
In PPCs (Plain Paper Copiers) and laser printers, printing is carried out with laser light of a laser diode. The laser light is irradiated onto for instance portions to be printed of a pre-charged roller to eliminate charges of the portions irradiated by the laser light. Toner is absorbed by the roller. Thereafter, the toner existing on the portions to which the laser light is irradiated is transferred onto paper. The transferred toner is printed on the paper by means of heat fusing.
Laser printing can give relatively crisp images and is suitable for high-speed printing. Accordingly, there is a strong demand for business use where a large volume and highspeed printing are necessary. Recently in particular, demand for high-speed printing is stronger.
In this case, transfer of printing data from an image processor to a laser driver becoming very fast, data transmission is switching from existing one due to TTL (Transistor-Transistor Logic) to another one due to LVDS (Low Voltage Differential Signal) suitable for high-speed data transmission.
US IEEE (The Institute of Electrical and Electronics Engineers, Inc.) has provided the LVDS as IEEE1596.3. As shown in FIG. 6, data transmission is executed with small amplitude differential signals.
That is, in the PPC and laser printer, as shown in FIG. 6, a LVDS transmitter 2, a LVDS receiver 3 and a cable 5 connecting therebetween, connects between an image processor 1 and a laser driver 4.
As shown in FIG. 7, when terminal resistance Rt connected to a differential input terminal of the LVDS receiver 3 is 100 xcexa9, an off-set voltage VOS of the differential input terminal of the LVDS receiver 3 is 1.125V to 1.275V, differential amplitude VOD being 0.25VP-P to 0.4VP-P, the upper limit of an output voltage VOHMAX being 1.475V, the lower limit of that VOLMIN being 0.925V.
Accordingly, relative to an existing TTL level of 5 VP-P, the amplitude being approximately one tenth thereof, data can be easily transmitted faster approximately ten times than that of the existing one, accordingly. Though the LVDS is suitable for the high-speed transmission, when used in the data transmission from the image processor to the laser driver, there occurs a big problem.
Ordinarily, for the PPCs and laser printers, from their structures, maintenance is indispensable. During maintenance operation, since the respective portions are necessary to be checked and adjusted in operation, it is necessary to open a cover without turning off the power source. At this time, in order to prevent an accident due to the laser light from occurring, the image processor 1 instructs the laser driver 4 to set data so that the laser does not emit. However, sometimes the cable connecting between the LVDS transmitter 2 and the LVDS receiver 3 may be disconnected.
Here, the input terminal of the LVDS receiver 3, being open, becomes indefinite as a state of input data. Accordingly, in the worst case situation, the laser is always in an emitting state to result in liability to serious accidents.
In order to avoid this from happening, the data transmission due to, other than the differential signal such as the LVDS, unbalanced signal such as the existing TTL signal can be implemented. However, the high-speed transmission becomes difficult.
Thus, when with the aforementioned existing LVDS, signal that drives the laser diode is transmitted/received, a serious accident may occur. On the other hand, when the unbalanced signal, instead of the LVDS, is used in the data transmission, the high-speed data transmission can not be realized.
The present invention is to provide a protecting unit that can prevent an accident, which becomes problems when the data are transmitted due to for instance the LVDS and based on the data for instance laser light is generated, from occurring.
To solve the aforementioned problems, the present invention discloses a protecting unit applied in an instrument comprising an input end to which differential signal, which is supplied to a differential input/output circuit, is transmitted, the input end being attachable to and detachable from an input line. The protecting unit comprises a circuit, and a comparator. The circuit sets a voltage of the input end when the input line is not connected to a voltage different from that generated at the input end when the input line is connected. The comparator compares a voltage of the input end with a reference voltage. Here, a state of output of the differential input/output circuit is fixed to a prescribed state based on the compared result.
Further, the present invention discloses a protecting unit applied in an instrument comprising an input end to which differential signal, which is supplied to a differential input/output circuit, is transmitted, the input end being attachable to and detachable from an input line. The protecting unit comprises a circuit, a node for detecting a voltage of the input end, and a comparator. The circuit sets a voltage of the input end when the input line is not connected to a voltage different from that generated at the input end when the input line is connected. The comparator compares a voltage at the node with a reference voltage. Here, a state of output of the differential input/output circuit is fixed to a prescribed state based on the compared result.
Still further, the present invention discloses a protecting unit applied in an instrument comprising an input end to which differential signal is transmitted, the input end being attachable to and detachable from an input line. The protecting unit comprises a bias circuit, a transmitter for transmitting variation of voltage at the input end to an input terminal of a differential input/output circuit, and a comparator. The bias circuit sets a voltage of the input end when the input line is not connected to a voltage different from that generated at the input end when the input line is connected. The comparator compares a voltage of the input terminal of the differential input/output circuit with a reference voltage. Here, a state of output of the differential input/output circuit is fixed to a prescribed state based on the compared result.
Thereby, an instrument comprising a differential input/output circuit that converts data inputted to a receiver input end that receives data transmitted from a transmitter transmitting differential signal to a prescribed output detects the input end of a receiver side to be opened. Thereby, if opened, output of the differential input/output circuit can be controlled.
Accordingly, when the present invention is applied in an electronic instrument using for instance laser, even if the cable is disconnected during maintenance operation, the laser can be instantly turned off to a non-emitting state. Thereby, occurrence of a serious accident due to laser light can be prevented.